Two-component coating compositions based on a hydroxyl-functional binder component and a polyisocyanate cross-linking agent are widely used in vehicle painting and industrial painting, particularly also in vehicle refinishing, owing to the very good technological properties of these coating compositions. The coating compositions are used in both water-based and solvent-based form.
Owing to tighter environmental law requirements, however, it is becoming increasingly necessary to use water-based coating compositions. Of course, paint manufacturers are anxious to provide such water-based coating, which are also at least on a par with solvent-based paints in terms of technological properties. Some effort was and still is required to this end, as solvent-based paint systems cannot automatically be switched 1:1 to water-based ones. For example, it is in the nature of water-based paints based on polyisocyanate cross-linking agents that secondary reactions occur between the water and the polyisocyanates. Owing to these secondary reactions, surface defects in the applied paint film may in turn occur. In this way, for example, so-called bubbles or pinholes may appear. It is known that the tendency for bubbles to form can be reduced by extending the flash-off time of the applied water-based coating. However, this lengthens the total process time, which in turn is not acceptable for certain applications, for example for vehicle refinishing applications.
A short drying time of the applied coating composition at moderate temperatures, for example from room temperature to approx. 60° C., is also necessary in vehicle refinishing applications. The drying and curing times may be considerably reduced by using catalysts for the cross-linking reaction. However, at the same time the use of catalysts generally also leads to a reduction of the pot life, i.e. to a reduction of the time within which the coating can still be processed and applied faultlessly.
Various approaches are known for eliminating the above-mentioned problem. For example, it is proposed to introduce the catalyst for the reaction between polyisocyanates and binders having groups capable of reacting with polyisocyanates not into the two-component coating composition or the layer of paint resulting therefrom itself, but rather into another layer of paint located thereabove or therebelow. In addition, U.S. Pat. No. 5,578,345 and WO 2007/068683, for example, disclose methods for multi-layer coating, wherein a clear coat composition based on a polyisocyanate cross-linking agent and a hydroxyl- and/or mercapto-functional binder is being applied in a multi-layer construction on a base coat layer which contains a catalyst for the cross-linking reaction in the clear coat. In this case, the catalyst is intended to migrate through the interface between the two coating layers and into the clear coat layer, after application of the clear coat composition, and therein to catalyze the cross-linking reaction. Since the two-component paint itself does not contain a catalyst, satisfactory processing times are achieved. However, a disadvantage of this method is that it can be difficult to control in a purposeful manner the migration of the catalyst into the layer of paint to be cross-linked, for example the layer of clear coat. As a result, over- or under-catalyzing can easily occur, which may for example lead to insufficient through-curing or to an unsatisfactorily long drying time.
Furthermore, it is known from WO 01/92362 to use photo-latent catalysts, for example amines, in coating compositions based on a polyisocyanate cross-linking agent and a mercapto-functional binder. The photo-latent catalyst is inactive in the coating itself and is only activated after the application of high-energy radiation by irradiation. In this way, an extended pot life can also be achieved. However, additional UV-lamp equipment is required for this purpose and sensitizers such as thioxanthones or benzophenones must be used in addition to the photo-latent catalysts.
Therefore, there remains a need for two-component coating compositions with a good balance between acceptable processing time (pot life) and short drying time, and in particular a need for water-based two-component coating compositions based on a binder component which is reactive with polyisocyanates and a polyisocyanate cross-linking agent, which result in perfect coats without surface defects in an acceptable total process time of the drying process. In addition, there remains a need for a water-based coating composition having a sufficient processing time and producing coating layers with very good technological properties, for example, very good hardness and resistance to chemicals and water. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.